The present invention relates to a coating composition and process that provide an activated coating on nickel screen. The coated nickel screen can be used as the cathode in an electrolytic cell that is designed for the generation of hydrogen and oxygen from an aqueous alkaline solution. A preferred coating is characterized by the presence of two activated layers with a high surface area, a multitude of fissures and a nickel to aluminum weight ratio greater than 20/1 in the top layer and greater than 4/1 in the bottom layer that is adjacent to the nickel substrate.
Activated nickel screens are currently being used for the synthesis of methane and the generation of hydrogen and oxygen in electrolytic cells containing an aqueous alkaline medium. In methane synthesis a mixture of carbon monoxide and hydrogen are passed over the activated nickel screens to form methane and water. In the production of hydrogen and oxygen in electrolytic cells, the activated nickel screens are used as the cathode. The activated screens, when used as the cathode in an electrolytic cell, lower the overvoltage and show more than a 20% improvement in efficiency over untreated nickel screens. It is believed that the superiority of the activated nickel screens is due, at least in part, to the increased surface area that results from the activation step. The activated screens have been used in electrolytic cells for the generation of hydrogen and oxygen for about ten years.
Hydrogen is presently being used as a fuel for industrial applications as well as a fuel for automobiles. The advantage of hydrogen as an automobile fuel include a greater energy release per unit weight of fuel and the absence of polluting emissions including carbon monoxide, carbon dioxide, nitrogen oxide, sulfur oxides, hydrocarbons, aldehydes, and lead compounds (i.e., the combustion products of hydrogen are primarily water with minute traces of nitrogen oxide).
The known process to produce the activated nickel screens included placing each individual nickel screen in a xe2x80x9cpackxe2x80x9d composed of a powder mixture containing aluminum, aluminum oxide and a halide salt activator followed by a heating operation (i.e., for several hours at elevated temperatures). This is known as the Classical Pack Cementation process and is disclosed in U.S. Pat. No. 4,349,612. The chemistry of this process during the heating step includes the reaction of the halide with aluminum to yield gaseous aluminum sub halide such as aluminum sub chloride (AlCl). As this gas passes over the nickel screen, it decomposes and deposits aluminum on the nickel surface. The process is carried out for 20 to 30 hours at 800-1200xc2x0 F. in a hydrogen atmosphere. At this temperature the deposited aluminum diffuses into the nickel surface to form a coating comprising an aluminum rich nickel aluminide (Ni2Al3). The process is labor intensive, requires long processing times, gives off obnoxious dusts during loading of the screens and emits corrosive and toxic halide gases during the heating operation. In order to prevent contamination of the environment, the effluent gases must be scrubbed under alkaline conditions to neutralize and remove the toxic gases. In addition, after each processing cycle, the coating powder must be sifted and replenished for the next load of screens. The powder mixture is sensitive to water absorption and must be kept dry when not in use. Otherwise the moisture will react with the activator in the pack and curtail its function.
After the formation of the nickel aluminide coating on the nickel screens, the screens are immersed in a 20% solution of sodium hydroxide for about 40-60 minutes at 180-200xc2x0 F. to selectively leach out at least a portion of the aluminum from the nickel aluminide coating. The screens are then rinsed in water and passivated by immersion for one hour in hot water at 180 to 212xc2x0 F. followed by a one hour immersion in a water solution containing 2-5% hydrogen peroxide at 74xc2x0 F. followed by rinsing in water and finally drying in an oven at 140-160xc2x0 F. to remove all water from the screen. After the foregoing processing, the screens are ready to be used as cathodes in electrolytic cells containing an aqueous alkaline medium (for example, 25% NaOH or 25% KOH in water). In these electrolytic cells, hydrogen is produced at the cathode and oxygen is produced at the anode. The anodes of the cells are usually composed of virgin (untreated) nickel. It is preferred that the anodes contain pores or openings (e.g., nickel screen).
The present invention includes the production of activated nickel screens with even greater activity than those produced by the aforementioned Classical Pack Cementation process. Further, the present invention includes a unique coating procedure which eliminates all of the disadvantages inherent in the Classical Pack Cementation process.
In the process of the present invention, the nickel screens are coated in a simple dipping procedure with a slurry of aluminum powder dispersed in a binder/organic solvent system or binder/water system. The coating must completely cover the surfaces of the wires that form the screen. After an initial drying step to remove the organic solvent or the water, the coating weight on the screen should not exceed about 30 mg/sqcm and should not be less than 10 mg/sqcm. The coated screen is next placed directly in a furnace under a nitrogen, hydrogen or inert atmosphere at a temperature of from about 1450-1750xc2x0 F. for a time of from about one to fifteen minutes. Coatings exceeding about 30 mg/sqcm will cause embrittlement of the wire during the heating operation. Coatings that are less than about 10 mg/sqcm will give an incomplete coating of the wires in the screen. During the heating step, aluminum is diffused into the surface of the nickel wires that form the screen where the aluminum reacts with the nickel to form nickel aluminides. By the end of the heating step, a coating has formed on the nickel wires. The portion of the coating that is closest to the external environment is predominantly NiAl3 and aluminum, whereas the portion of the coating that is closest to the nickel wire is predominantly Ni2Al3 and nickel. Subsequent leaching of the aluminum from this coating in a water solution containing 20% sodium or potassium hydroxide at 180-200xc2x0 F. provides a coating with greater activity than the coating that is formed in the Classical Pack Cementation process which does not have the same structure as the coating of the present invention. In addition to its greater activity, the process of the present invention offers substantial cost savings in labor and the elimination of the release of obnoxious dusts and toxic gases during the coating and heating steps. In addition, as is shown in FIG. 1, there is a marked improvement in the performance of the electrolytic cells that used the activated nickel screens produced by the process of the present invention compared to the activated nickel screens produced by the Classical Pack Cementation process. This improvement in properties is believed to be a result of the differences in structure and composition between the coating of the present invention (i.e., the coating formed on the nickel screens) and the coating formed by the Classical Pack Cementation process. Specifically, the coating of the present invention (i.e., when viewed at 800xc3x97 magnification) appears to have two parts or sections, see FIGS. 3 and 5. The outer part or section has a serrated appearance with the points of the toothlike projections facing outward (i.e., towards the external environment). The nickel to aluminum ratio (by weight) in this outer part or section of the coating is at least 20 to 1. The inner part or section, which is contiguous with the nickel wire of the screen, has the appearance of a substantially solid or uniform layer that is interlaced with fissures or cracks. The nickel to aluminum ratio (by weight) in this inner part or section of the coating is at least 4 to 1. In contrast, the coating that is formed by the Classical Pack Cementation process has only one part or section which has the appearance of a solid or uniform layer (see FIGS. 2 and 4). Further, the coating that is formed by the Classical Pack Cementation process does not have as many fissures or cracks as the inner part or section of the coating that is formed by the process of the present invention. Finally, the coating that is formed by the Classical Pack Cementation process (before the leaching step) is composed predominantly of Ni2Al3. After the leaching step, the ratio of nickel to aluminum in the coating is about 3.3 to 1.
The two part structure of the coating of the present invention in combination with the increased number of fissures or cracks in the coating result in an increased surface area that is available for interaction with the external environment. In addition, the coating of the present invention also has a higher nickel to aluminum ratio than the coating formed by the Classical Pack Cementation process. The combination of these differences results in an activated nickel screen (i.e., the screen of the present invention) that has superior properties (e.g., superior catalytic properties) than the activated nickel screen that is produced by the Classical Pack Cementation process.
Another major advantage of the innovative coating process of the present invention over the Classical Pack Cementation process is that the process of the present invention can be run continuously whereas the Classical Pack Cementation process was a labor intensive batch process. Specifically, because the process of the present invention utilizes: (1) a simple dip-coating procedure to coat the nickel screen with aluminum powder and (2) a short heating step, it is possible to run the process in a continuous manner where a coiled screen is slowly uncoiled and first fed through a dip-coating station where the screen is coated with an aluminum containing slurry and then the slurry coated screen is passed through a heating unit where the liquid component of the slurry is removed before the screen is passed through a furnace to cause the aluminum powder to diffuse into the surface of the nickel wire that makes up the screen. The use of a powder bed in the initial coating step and the extremely long processing times required for the Classical Pack Cementation process would preclude such continuous processing.
The average particle size of the aluminum powder that is used to form the slurry of aluminum powder in the process of the present invention should be smaller than 40 microns but larger than about 5 microns. Too small a particle size will cause premature melting and run off of the aluminum from the screen during the heating operation, whereas too large a particle size will result in incomplete coating of the screen. The preferred particle size for the aluminum powder is between 5 and 20 microns.
During the leaching step, aluminum is removed from the nickel aluminides in the coating by the same process that is used in the Classical Pack Cementation process. Specifically, the coated nickel screens are immersed in a solution containing about 20% by weight sodium or potassium hydroxide in water for about one hour at a temperature of about 180-212xc2x0 F., preferably about 200-212xc2x0 F.
The major advantage in processing using the innovative process of the present invention is in the coating procedure. By using the process of the present invention, a substantial reduction in cost can be realized through a reduction in the labor costs associated with the Classical Pack Cementation process. In addition, by using the process of the present invention, it is possible to drastically reduce or even eliminate the release of obnoxious and harmful dusts and gaseous effluents into the environment that was associated with the Classical Pack Cementation process.